1. Field of the Invention
The present invention relates to a lubricating fluid composition for dynamic pressure bearings, which fluid supports the shaft of the bearing by dynamic pressure, and to the magnetic characteristics of the fluid which are utilized for sealing action.
2. Description of the Background
In recent years, attention has been directed to the bearing component of such devices as motors by which various kinds of rotating members such as a polygon mirror, magnetic disc, optical disc, and the like are rotated at high speed. Attention has been directed to the use of a magnetic lubricating fluid as a lubricating fluid for the dynamic pressure bearing. That is, a magnetic fluid is a colloidal solution in which fine ferromagnetic particles are stably dispersed in a liquid solvent. In view of the strong ferromagnetic property which the liquid itself shows, the fluid is usually used as a sealant in combination with ball bearings in many cases. However, dynamic pressure bearings which use the magnetic fluid as a lubricating fluid of the bearing have great prospects, because the dynamic pressure bearing is superior in high speed rotation stability and stillness, and the like, in comparison to ball bearings. Dynamic pressure bearings provided with a magnetic fluid have been proposed and have great prospects. Dynamic pressure bearings provided with a magnetic fluid have been proposed and disclosed in, for example, Japanese Laid-Open Patent Application No. 88223/1985, which discloses a device having a fluid which acts as both a lubricating fluid for generating dynamic pressure and a magnetic fluid for magnetic sealing.
A magnetic fluid composition has been used in the past for sealing purposes, because of its most important property for a sealing, which is its most noteworthy property, which is that of high sealing at pressures of 10.sup.-6 Torr under high speed rotation conditions. However, a magnetic fluid composition which simultaneously satisfies both functions as a lubricating fluid and a sealing magnetic fluid (low abrasion characteristics and good lifetime characteristics of low volatility and high temperature resistance) has not yet been developed (see PETROTECH, vol. 13, No. 12 (1989)).
However, in such a device as mentioned above, the magnetic fluid composition must have both a low volatility as a seal and a low viscosity for lowering bearing loss. That is, the bearing portion of a device is exposed to far greater shearing stress and higher temperature than the sealing portion of a bearing device. Sometimes, active metal surfaces are formed by metal-metal contact and wear in the bearing, particularly at starting or stopping of a bearing containing device, with which the fluid comes into contact.
Thus, there is a need for a lubricating fluid composition for dynamic pressure bearings, which achieves not only high sealing properties, but also high lubricity and long lifetime characteristics. More specifically, the characteristics which should be possessed by a lubricating fluid composition for use as both a lubricating fluid for dynamic pressure generation and a magnetic fluid for sealing, are:
1) a low viscosity of 100 CP or less (at 27.degree. C.), PA1 2) a saturation magnetization of 50 gauss or more, PA1 3) low volatility which can maintain sealing characteristics, PA1 4) high temperature gelation resistance, oxidation resistance, and the like. (The fluid exhibit stability while suppressing these factors.) and PA1 5) restrain the activity of active surface metal caused by contact or wear of metal surfaces.
A magnetic fluid satisfying these characteristics has not yet been obtained. Above all the lifetime characteristics of the magnetic lubricant fluid, the phenomenon of so-called gelation, in which fluidity is lost, particularly at high temperatures, is a serious problem which needs to be solved.
It has been known to add 0.1 to 0.5% by weight of a phenolic, amine or sulfur containing antioxidant to a lubricating oil in order to improve the effective lifetime of the fluid. However, nothing has been known as to what kind of material(s) could be added to the fluid in what kind of manner, to prevent gelation of a magnetic fluid. Japanese Laid-Open Patent Application No. 105093/1984, discloses that an antioxidant can be added to the fluid in order to prevent deterioration of the magnetic fluid by oxidation, but there is no disclosure about prevention of gelation. A need therefore continues to exist for a magnetic fluid which exhibits superior properties, particularly superior gelation resistance.